The present invention relates to a flux for brazing difficult to wet metallic stock materials with silver- and copper-based solders. Metallic stock materials of this kind which are difficult to wet by the solder in the brazing process are substantially stainless and scale-free steels and composite materials produced by powder metallurgy, such as carbide metals.
Soldering is a thermal process for joining and coating metallic stock materials or metallic workpieces by fusion, wherein a liquid phase is obtained by melting a metallic material, namely the solder. The solidus temperature of the stock materials is not reached during this process. As used herein the terms xe2x80x9cworkpiecexe2x80x9d and xe2x80x9cstock materialxe2x80x9d are interchangeable.
Solders used are alloys or pure metals. If solders are processed whose liquidus temperature is above 450xc2x0 C., the term brazing is used.
Assuming pure metal surfaces, the liquid solder is able to wet the metallic stock material in so far as it forms mixed crystals or intermetallic compounds with said stock material. The solder then spreads out over the joint surface and, after solidifying, forms a loadable joint with the metallic stock material.
If designed in a manner suitable for soldering, the two joint surfaces of the parts to be joined form a narrow parallel slit. The molten solder then flows of its own accord into the soldering slit due to the capillary filling pressure taking effect, and fills said slit. The minimum temperature on the surface of the component to be soldered at which said process takes place undisturbed is the so-called working temperature. It is a characteristic quantity for the solder in question.
In order to be able to form a bond with the metallic stock material, the molten solder must come into direct contact with the metallic stock material. Oxide layers of the kind present on any engineering metal surface must thus be loosened first and removed. If soldering takes place in the air, this is achieved by covering the soldering site with fluxes in the melt flow of which the oxides dissolve or are decomposed at soldering temperature.
The flux thus primarily has the task of removing oxides present on the solder and stock material surfaces and preventing them from re-forming during the soldering process so that the solder is able to wet the stock material sufficiently.
The melting point and the effective temperature of the fluxes must be matched to the working temperature of the solder used, whereby the flux should melt at about 50-100xc2x0 below the working temperature of the solder used and become fully effective from this temperature onwards. Moreover, the molten flux should form a dense, uniform coating on the workpiece which remains intact at the required soldering temperature and for the duration of the soldering period.
Brazing fluxes are composed substantially of salt mixtures which, in the molten state, are capable of dissolving metal oxides. Said fluxes are substantially inorganic boron compounds such as, in particular, alkali borates and fluoroborates, and halides such as, in particular, an alkali halide; e.g. alkali fluorides.
The standard DIN EN 1045 classifies fluxes for brazing heavy metals (FH type) into seven classes according to their composition and effective temperature. The fluxes are used as powders, pastes or suspensions, the latter being applied to the workpieces by spraying, brushing or dipping. They are then heated to melting point and the workpiece materials are joined together by fusion by adding solder. It is also known to combine flux and solder in one product. Flux-coated moulded parts or flux-covered solder wires are thus also used in practice for joining the workpieces or stock materials.
DE 24 44 521 describes a flux for soldering which is composed of boric acid and various alkali metal polyborates. Said flux may also additionally contain 1 wt. % of boron in elemental form.
GB 909 314 discloses a brazing flux for brazing nickel and nickel alloys. Said flux contains, apart from the conventional constituents such as potassium tetrafluoroborate, potassium metaborate and potassium fluoride, additionally copper compounds such as copper oxide and copper chloride. The latter constituents are claimed to suppress the reaction of the flux with the stock material in order to prevent the latter becoming brittle.
The well known fluxes have a major disadvantage, however. Difficult to wet stock materials such as, for example, stainless and scale-free steels and carbide metals, particularly carbide metals with low cobalt contents of less than 6% (K01-K10, PO5) are wetted only insufficiently by the liquid solder when the conventional fluxes are used. Fluxes containing an addition of elemental boron (FH12 type) are used in engineering to join said stock materials. The addition of boron is designed to increase the stability above about 700xc2x0 C., that is, the time during which the flux is active. The wetting of carbide metals and chromium-nickel steels themselves is not, however, improved in practice by the addition of boron, particularly when soldering with low working temperatures (below 680xc2x0 C.).
Special soldering tasks, such as, for example, the joining of diamond-impregnated carbide metals require further reduced soldering temperatures so that the diamond filling and the carbide metal are not damaged. Cadmium-containing silver brazing solders or preferably gallium-containing silver brazing solders as described, for example, in DE 43 15 188 and DE 43 15 189, are used for said application. Said solders have a working temperature from 590xc2x0 C. to 640xc2x0 C. Conventional fluxes have only insufficient effectiveness and wetting ability in this temperature range. This reduces the process reliability during soldering and thus leads to increasing scrap and reduced product quality.
An object of the present invention is, therefore, to develop a flux for brazing difficult to wet stock materials such as stainless and scale-free steels and carbide metals, wherein all combinations of salts provided for by the standard DIN EN 1045 for brazing heavy metals (FH type)are used as the basic constituents of said flux, and which, with silver- and copper-based solders, permits a markedly improved wetting of the stock materials.
The above and other objects of the invention can be achieved by a flux based on inorganic boron and/or halogen compounds which contains, as an activating addition, based on the total amount of flux, 0.01-10 wt. % of elemental boron and 0.01-10 wt. % of at least one of the elements Mo, W, Mn, Ni, Pd, Cu or Ag in the form of elements, alloys or compounds.
Surprisingly, it has become apparent that as a result of the flux according to the invention, the wetting of difficult to wet materials such as stainless and scale-free steels and more particularly also carbide metals and hard materials with low metal contents is markedly improved compared with conventional fluxes. Evidently, the content of boron and additionally of at least one of the other elements brings about a corresponding activation of fluxes with an otherwise conventional composition. It is interesting in this connection that the individual constituents of said activator mixture on their own do not exhibit an improvement in the effectiveness of the conventional fluxes. Only the combination of boron with at least one of the elements, an alloy or compound of Mo, W, Mn, Co, Ni, Pd, Cu and Ag exhibits this surprising behavior in conjunction with brazing fluxes of boron and/or halogen compounds.
The brazing flux according to the invention is based substantially on conventional basic substances known for such fluxes and the compositions thereof in terms of quantity, as specified, for example, in terms of quality and quantity by the standard DIN EN 1045.
Basic constituents of such kind are inorganic boron and/or halogen compounds. Suitable boron compounds are primarily boric acid and borates or complex borates of alkali and alkaline earth metals, particularly borax (sodium tetraborate), potassium tetraborate and potassium tetrafluoroborate. Suitable halogen compounds are primarily fluorides and chlorides of alkali and alkaline earth metals, particularly potassium and sodium fluoride or potassium and sodium hydrogen fluoride.
According to the invention, the brazing flux contains an activator combination of, based on the total amount, 0.01-10 wt. % of elemental boron and 0.01-10 wt. % of at least one of the elements Mo, W, Mn, Co, Ni, Pd, Cu or Ag in order to intensify the wetting. The flux preferably contains 0.1-5 wt. % of elemental boron and 0.1-5 wt. % of the other elements mentioned, in each case based on the total amount of flux.
Advantageously, the elemental boron used is boron in amorphous form. The other elements may be used on their own or in combination in elemental form, in the form of alloys containing them, or in the form of compounds thereof. Suitable compounds are preferably corresponding oxides.
The components of the activator combination are used advantageously in the finely powdered form, in which case the average particle size thereof should preferably be less than 45 xcexcm.
The flux according to the invention for brazing difficult to wet materials is prepared by mixing, intensively and homogeneously, based on the total amount of flux, initially 0.01-10 wt. % of finely powdered boron powder with 0.01-10 wt. % of a finely powdered addition of at least one of the elements Mo, W. Mn, Co, Ni, Pd, Cu and Ag, which may be present in the form of elements, alloys or compounds, and then adding this activator mixture afterwards to the finely powdered flux mixture of inorganic boron and halogen compounds.
Flux mixtures which, in addition to boron, contain the elements tungsten or manganese in elemental form, as an alloy, or in the form of an oxide, are particularly effective.
The brazing flux according to the invention preferably contains, as activator mixture, based on the total amount of flux, 0.5-3 wt. % of boron in amorphous form and 0.5-3 wt. % of manganese with an average particle size in each case of less than 45 xcexcm. The manganese may be used here in powder form as a metal, as an alloy or as an oxide compound without the effectiveness being affected as a result. Advantageously, the manganese is introduced in the form of the alloy MnNi40 or as manganese dioxide.
In order to adjust to certain stock materials and operating conditions, the flux may contain up to 2 wt. % of Si preferably in the form bound to oxygen, for example, as silicon dioxide. This additive, too, is added to the mixture in a correspondingly finely powdered form.
The flux according to the invention is initially present as a powder mixture and may already be used as such in an inherently known way.
The flux may also be present in the form of pastes or suspensions, for which the powder mixture is converted to a paste and/or suspension with inert liquids such as water, aliphatic alcohols, glycols etc.
Such flux pastes or suspensions may also contain minor amounts of auxiliary substances such as, e.g., conventional surfactants, binders or thickeners.
For application, the brazing flux according to the invention is applied to the stock material surfaces to be joined or soldered in a manner suitable for the form in question, for example, by sprinkling, coating, brushing, spraying or dipping, and the brazing process is carried out. In so doing, the flux according to the invention brings about a substantial improvement in the wetting behavior of the solder on the stock materials during brazing with silver- and copper-based solders. This is particularly pronounced and advantageous with difficult to wet metallic stock materials such as, in particular, stainless and scale-free steels and composite materials produced by powder metallurgy. The latter include carbide metals and hard materials of metal-ceramic composite materials (e.g. so-called xe2x80x9ccermetsxe2x80x9d), more particularly those with a low metal content.
The flux according to the invention may also be combined with soldering materials, for example, as a powder mixture, as a paste or suspension, which contain, in addition, fine-particle brazing solder in each case.
Other solder-flux combinations are, for example, flux-coated solders which may be obtained, for example, by extrusion of drying or curing flux paste formulations on solder moulded parts such as rods and rings.